Pressure sensors or pressure transducers can be utilized in a wide range of sensing applications. In many cases, it is desirable to measure the pressure of a particular type of media (e.g., usually gases or liquids) such as water, fuel, oil, acids, bases, solvents and corrosive gases. The sensed media can also include (but need not be limited to) air, nitrogen, industrial process gases, water, automotive fluids, pneumatic fluids, coolants, industrial chemicals, etc. For such applications, pressure sensors can be utilized to accurately sense the pressure of the media.
In particular, one type or configuration of pressure sensor is referred to as a differential pressure sensor. This type of sensor measures the difference between two or more pressures that are supplied as inputs. An example application for a differential pressure sensor may involve measuring the pressure drop across a furnace filter or an oil filter to determine the level of clogging. Another differential pressure sensor application may be implemented in conjunction with the venturi effect to measure flow. In such a situation, a pressure differential can be created between two segments of a venturi tube that are designed with a different aperture. The pressure difference is directly proportional to the flow rate through the venturi tube and can be accurately measured by a differential pressure sensor.
One of the most common types of differential pressure sensors utilized in many industrial and commercial applications is a solid-state MEMS pressure sensor that utilizes silicon piezoresistive technology. A typical MEMS pressure die employs a thin silicon diaphragm that is stressed in response to an applied pressure. Piezoresistors are strategically located or positioned on the diaphragm. When pressure is applied to the sensor, the diaphragm is stressed and the piezoresistors convert this mechanical stress to an electrical signal. Typically, the piezoresistors form a Wheatstone bridge and the differential signal is proportional to the applied pressure.
In wet applications, the pressure sensor comes into contact with liquids or with gases having high moisture content. The differential pressure sensors can also be utilized in wet/wet applications where both sides of the sense die are exposed to the sensed media such as water or oil. Such differential pressure sensors can require the fluid media on both the top and bottom sides of the diaphragm. Hence the diaphragm of the differential pressure sensor can come into contact with the media that can be usually corrosive or harmful. This corrosive or harmful media can damage components of the pressure sensors, in particular bond pads that are exposed electrical connection to the differential pressure sensor.
In many applications, even if the media itself is not electrically conducting it can create a harsh environment for the exposed bond pads, resulting in long-term reliability failures. Hence, it is desirable to isolate sensing elements, circuitry and electrical connections from direct contact with the media for reliable operation. However there are very few cost effective solutions for a wet-wet pressure sensor. Most solutions are based on stainless steel isolation diaphragm design construction utilizing oil filled media isolated silicon piezoresistive technology. In some isolation arrangements of the differential pressure sensors, the environmentally sensitive silicon pressure die can be sandwiched between elastomeric seals one of which includes a conductive stack for electrical connection. The pressure sensors can utilize the pre-molded elastomeric seals to separate the pressure die from a relatively harsh, wet, pressure sensing environment. Such pressure sensors can obtain a true differential operation and an accurate pressure of the media, but the increased production and material costs can be prohibitive. Also, one problem which can be associated with such pressure sensors is thermal hysteresis associated with the elastomeric seals, which flex and move over temperature, thus causing shifts in the parametric performance of the device.
A need therefore exists for an improved differential pressure sensor with high reliability for wet/wet applications, which can provide media isolated electrical connections that are ultimately more efficient and robust than presently implemented pressure sensors. Such differential pressure sensors are described in greater detail herein.